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Atoms, Molecules and Mole Concept PYQs – Chemistry, SSC & Railway

100 Atoms, Molecules and Mole Concept (Chemistry) questions asked in recent SSC and Railway exams, newest first, with exam, date & shift, answers and solutions.

📚 100 questionsSSC 3 · Railway 97
RRB Group D 2025-26 – 25 August 2026 Shift 1 · Dalton's atomic theory

Q1. What does Dalton’s atomic theory suggest about atoms of different elements?

  1. (a) They have different sizes but identical masses.
  2. (b) They have different masses and properties.
  3. (c) They have identical masses but different shapes.
  4. (d) They are the same in every aspect.
Answer & solution

Answer: (b) They have different masses and properties.

Correct Answer:
Correct Answer: Option 2 – They have different masses and properties.
Explanation:
According toDalton's atomic theory, atoms of the same element are identical in mass and properties, whereasatoms of different elements have different masses and different chemical properties. This was one of the foundational postulates that explained why elements combine in fixed proportions.
Important Facts:
• Dalton proposed atoms are indivisible and indestructible (later modified with discovery of subatomic particles).
• Atoms of the same element are alike in all respects, per Dalton's theory.
• Atoms combine in simple whole-number ratios to form compounds.
RRB Group D 2025-26 – 25 August 2026 Shift 1 · Atomic mass and molecular mass

Q2. Which of the following elements has molecules with the highest number of atoms?

  1. (a) Oxygen
  2. (b) Nitrogen
  3. (c) Sulphur
  4. (d) Chlorine
Answer & solution

Answer: (c) Sulphur

Correct Answer:
Correct Answer: Option 3 – Sulphur
Explanation:
Among the given options,Sulphurexists as anS₈molecule (a ring of 8 sulphur atoms), which has the highest number of atoms per molecule. Oxygen (O₂) and Nitrogen (N₂) are diatomic, and Chlorine (Cl₂) is also diatomic — all having only 2 atoms per molecule.
Important Facts:
• Phosphorus also forms polyatomic molecules (P₄), but Sulphur's S₈ has more atoms.
• Ozone (O₃), an allotrope of oxygen, has 3 atoms, still fewer than S₈.
• Molecularity here refers to atomicity — the number of atoms constituting one molecule of the element.
RRB Group D 2025-26 – 25 August 2026 Shift 2 · Ions and radicals

Q3. Which of the following statements is/are correct?Statement I: Sodium atom loses one electron to form a sodium ion.Statement II: Sodium ion has a negative charge.

  1. (a) Only Statement II is correct.
  2. (b) Both statements I and II are correct.
  3. (c) Only Statement I is correct.
  4. (d) Both statements I and II are incorrect.
Answer & solution

Answer: (c) Only Statement I is correct.

Correct Answer:
Correct Answer: Option 3 – Only Statement I is correct.
Explanation:
Statement I is correct: A sodium atom (Na, atomic number 11, electronic configuration 2,8,1) loses its single valence electron to attain a stable octet configuration, forming a sodium ion (Na⁺).Statement II is incorrect: since sodium loses an electron, the resulting sodium ion carries apositive charge (Na⁺), not a negative charge.
RRB Group D 2025-26 – 25 August 2026 Shift 3 · Laws of chemical combination

Q4. Which of the following observations would violate the law of constant proportions?

  1. (a) A sample of sodium chloride with sodium and chlorine in a 23:35.5 mass ratio.
  2. (b) A sample of water containing hydrogen and oxygen in a 1:8 mass ratio.
  3. (c) A sample of magnesium sulphide containing magnesium and sulphur in a 4:3 mass ratio.
  4. (d) A sample of ammonia containing nitrogen and hydrogen in a 14:3 mass ratio.
Answer & solution

Answer: (c) A sample of magnesium sulphide containing magnesium and sulphur in a 4:3 mass ratio.

Correct Answer:
Correct Answer: Option 3 – A sample of magnesium sulphide containing magnesium and sulphur in a 4:3 mass ratio.
Explanation:
The Law of Constant Proportions states that a chemical compound always contains the same elements combined in a fixed mass ratio, regardless of source. Checking each option: NaCl (Na=23, Cl=35.5) gives 23:35.5 — correct actual ratio. Water (H2=2, O=16) gives 1:8 — correct ratio. Ammonia (N=14, H3=3) gives 14:3 — correct ratio. But MgS (Mg=24, S=32) has an actual mass ratio of 24:32 = 3:4, not 4:3, so a sample showing 4:3 would violate the law.
Important Facts:
• Law of Constant Proportions was given by Joseph Proust.
• It applies regardless of the source or method of preparation of the compound.
• It underlies the concept of a fixed chemical formula for every compound.
RRB Group D 2025-26 – 25 August 2026 Shift 3 · Laws of chemical combination

Q5. Which of the following statements is/are CORRECT?Statement I: Atoms of different elements combine in definite proportions to form compounds.Statement II: In a pure chemical compound, elements are always present in a definite proportion by mass.

  1. (a) Both statements I and II are correct
  2. (b) Only Statement II is correct
  3. (c) Both statements I and II are incorrect
  4. (d) Only Statement I is correct
Answer & solution

Answer: (a) Both statements I and II are correct

Correct Answer:
Correct Answer: Option 1 – Both statements I and II are correct
Explanation:
Both statements are restatements of the Law of Constant (Definite) Proportions: elements in a compound always combine in a fixed, definite mass ratio, irrespective of the source or method of preparation of the compound.
Important Facts:
• This law was proposed by Joseph Proust.
• It applies to pure compounds, not mixtures.
• It is one of the foundational laws of chemical combination, alongside the Law of Conservation of Mass.
RRB Group D 2025-26 – 21 August 2026 Shift 1 · Laws of chemical combination

Q6. A sample of water obtained from two different sources is found to contain hydrogen and oxygen in the ratio 1:8 by mass. Which law is illustrated by this observation?

  1. (a) Law of reciprocal proportions
  2. (b) Law of conservation of mass
  3. (c) Law of multiple proportions
  4. (d) Law of constant proportions
Answer & solution

Answer: (d) Law of constant proportions

Explanation:
The Law of Constant (Definite) Proportions states that a pure chemical compound always contains the same elements combined together in the same fixed proportion by mass, regardless of the source or method of preparation. Since water from two different sources shows the same 1:8 mass ratio of hydrogen to oxygen, this law is illustrated.
Correct Answer: Option 4 – Law of constant proportions
RRB Group D 2025-26 – 21 August 2026 Shift 2 · Atomic mass and molecular mass

Q7. What does the formula unit mass of a substance represent?

  1. (a) Ratio of atomic masses of metal and nonmetal atoms
  2. (b) Difference between molecular and atomic masses
  3. (c) Product of atomic numbers of all atoms in a molecule
  4. (d) Sum of atomic masses of all atoms in a formula unit
Answer & solution

Answer: (d) Sum of atomic masses of all atoms in a formula unit

Correct Answer:
Correct Answer: Option 4 – Sum of atomic masses of all atoms in a formula unit
Explanation:
For ionic compounds (which exist as formula units rather than discrete molecules, e.g., NaCl), the formula unit mass is calculated the same way as molecular mass — by adding up the atomic masses of all atoms present in one formula unit.
RRB Group D 2025-26 – 20 August 2026 Shift 2 · Atomic mass and molecular mass

Q8. Which of the following is a correct statement about molecules of elements?

  1. (a) They are always monoatomic.
  2. (b) They can only exist in solid form.
  3. (c) They consist of two or more atoms of the same element.
  4. (d) They consist of two or more atoms of different elements.
Answer & solution

Answer: (c) They consist of two or more atoms of the same element.

Correct Answer:
Correct Answer: Option 3 – They consist of two or more atoms of the same element.
Explanation:
A molecule of an element is formed by the combination of two or more atoms of thesameelement, e.g., O₂, N₂, P₄, S₈. Molecules of compounds, by contrast, are formed from atoms of different elements.
Important Facts:
• Noble gases (He, Ne, Ar) exist as monoatomic molecules, so 'always monoatomic' is false.
• Elemental molecules can exist as gas, liquid, or solid depending on conditions.
RRB Group D 2025-26 – 19 August 2026 Shift 1 · Atomic mass and molecular mass

Q9. The formula unit mass of ammonium sulphate, (NH₄)₂SO₄, is:

  1. (a) 132 u
  2. (b) 114 u
  3. (c) 118 u
  4. (d) 136 u
Answer & solution

Answer: (a) 132 u

Correct Answer:
Correct Answer: Option 1 – 132 u
Explanation:
Formula unit mass of (NH₄)₂SO₄ = 2×(N) + 8×(H) + 1×(S) + 4×(O) = 2(14) + 8(1) + 32 + 4(16) = 28 + 8 + 32 + 64 =132 u.
Important Facts:
• Atomic masses used: N = 14 u, H = 1 u, S = 32 u, O = 16 u.
• Formula unit mass is used for ionic compounds (instead of molecular mass).
RRB Group D 2025-26 – 19 August 2026 Shift 1 · Ions and radicals

Q10. What is formed when a non-metal gains electrons?

  1. (a) Cation
  2. (b) Neutral
  3. (c) Anion
  4. (d) Proton
Answer & solution

Answer: (c) Anion

Correct Answer:
Correct Answer: Option 3 – Anion
Explanation:
When a non-metal atomgains electrons, it acquires a netnegative charge, forming a negatively charged ion called ananion(e.g., Cl⁻, O²⁻). Losing electrons instead produces a positively chargedcation.
Important Facts:
• Metals typically lose electrons to form cations.
• Non-metals typically gain electrons to form anions, achieving a stable octet.
RRB Group D 2025-26 – 19 August 2026 Shift 2 · Ions and radicals

Q11. Which statement best describes the result when a sodium atom in the gaseous state loses one electron?

  1. (a) The atom becomes a sodium ion with a positive charge.
  2. (b) The atom becomes a sodium ion with a negative charge.
  3. (c) The atom is converted into a neutron.
  4. (d) The atom remains a neutral sodium atom.
Answer & solution

Answer: (a) The atom becomes a sodium ion with a positive charge.

Correct Answer:
Correct Answer: Option 1 – The atom becomes a sodium ion with a positive charge.
Explanation:
A neutral sodium atom (Na) has equal numbers of protons and electrons. When it loses one electron, the number of protons (11) exceeds the number of electrons (10), giving the resulting species a net+1 charge. This is the sodium cation, Na⁺.
Important Facts:
• Losing electrons → cation (positive charge); gaining electrons → anion (negative charge).
• Na → Na⁺ + e⁻ is the ionization of sodium.
• Protons never change in a simple electron loss/gain, so the atom is not converted into a different element or a neutron.
RRB Group D 2025-26 – 19 August 2026 Shift 3 · Valency and chemical formulae

Q12. An element X forms a chloride with the formula XCl₃ and an oxide with the formula X₂O₃. Based on this, what is the valency of element X?

  1. (a) 1
  2. (b) 2
  3. (c) 3
  4. (d) 4
Answer & solution

Answer: (c) 3

Correct Answer:
Option 3 – 3
Explanation:
In XCl₃, three chlorine atoms (each with valency 1) combine with one X atom, so X has a valency of 3. Similarly, in X₂O₃, oxygen has a valency of 2; balancing charges (2×valency of X = 3×2 = 6, so valency of X = 3) confirms the valency of X is3.
Important Facts:
• Valency is the combining capacity of an element, determined by the number of electrons gained, lost, or shared.
• Elements like Al (Aluminium) commonly show a valency of 3, forming AlCl₃ and Al₂O₃.
Correct Answer: Option 3 – 3
RRB Group D 2025-26 – 18 August 2026 Shift 1 · Valency and chemical formulae

Q13. The importance of valency in forming chemical formulae lies in:

  1. (a) Predicting the energy released during compound formation
  2. (b) Measuring the reactivity rate of atoms in gaseous condition
  3. (c) Determining how atoms of elements combine with each other
  4. (d) Identifying the molecular geometry using bonding angles only
Answer & solution

Answer: (c) Determining how atoms of elements combine with each other

Role of valency
Valency is the combining capacity of an atom, determined by the number of electrons in its outermost shell that it can lose, gain, or share.
It is used to determine how many atoms of one element combine with atoms of another element to form a chemical compound, which allows us to write correct chemical formulae (e.g., using the criss-cross method).
Correct Answer: Determining how atoms of elements combine with each other
RRB Group D 2025-26 – 18 August 2026 Shift 2 · Laws of chemical combination

Q14. Which observation would show a violation of the law of conservation of mass in a closed system?

  1. (a) A white precipitate forms after mixing solutions
  2. (b) The color of the solution changes after reaction
  3. (c) The mass of the products is less than the mass of the reactants
  4. (d) The temperature of the system increases
Answer & solution

Answer: (c) The mass of the products is less than the mass of the reactants

Law of Conservation of Mass
In a closed system, the law of conservation of mass states that total mass of reactants must equal total mass of products — no matter is lost.
If the observed mass of products isless thanthe mass of reactants in a closed system, that would directly contradict/violate this law (mass changes due to colour change or temperature rise do not violate mass conservation).
Correct Answer: The mass of the products is less than the mass of the reactants
RRB Group D 2025-26 – 17 August 2026 Shift 2 · Valency and chemical formulae

Q15. Which statement correctly describes the valency exhibited by phosphorus?

  1. (a) Phosphorus can exhibit valency of three or five
  2. (b) Phosphorus exhibits only a valency of five.
  3. (c) Phosphorus exhibits only a valency of three.
  4. (d) Phosphorus exhibits only a valency of two.
Answer & solution

Answer: (a) Phosphorus can exhibit valency of three or five

Valency of Phosphorus
Phosphorus has the electronic configuration 2,8,5, and it shows variable valency. It commonly exhibits avalency of three (as in PCl₃) or five (as in PCl₅), owing to the availability of d-orbitals allowing expansion of its valence shell.
• It is not restricted to only three, only five, or only two.
Correct Answer: Phosphorus can exhibit valency of three or five
RRB Group D 2025-26 – 14 August 2026 Shift 2 · Atomic mass and molecular mass

Q16. Among the following, what has a molecular mass of 44 u?

  1. (a) CO₂
  2. (b) H₂O
  3. (c) N₂
  4. (d) NH₃
Answer & solution

Answer: (a) CO₂

Answer: Option 1 – CO₂.
• CO₂ = 12 + 2×16 =44 u.
RRB Group D 2025-26 – 13 August 2026 Shift 2 · Laws of chemical combination

Q17. Why is the reaction flask corked during experiments verifying conservation of mass?

  1. (a) To prevent loss of matter
  2. (b) To increase reaction speed
  3. (c) To cool reaction mixture
  4. (d) To allow air circulation
Answer & solution

Answer: (a) To prevent loss of matter

Answer: Option 1 – To prevent loss of matter.
• Corking stops gases escaping, sono matter is lost– confirming conservation of mass.
RRB Group D 2025-26 – 13 August 2026 Shift 3 · Valency and chemical formulae

Q18. What is the valency of oxygen in MgO?

  1. (a) Two
  2. (b) Three
  3. (c) Four
  4. (d) One
Answer & solution

Answer: (a) Two

Answer: Option 1 – Two.
• In MgO, oxygen has valency2(O²⁻).
RRB Group D 2025-26 – 12 August 2026 Shift 1 · Atomic mass and molecular mass

Q19. What is the formula unit mass of calcium chloride (CaCl₂)?

  1. (a) 71 u
  2. (b) 46 u
  3. (c) 65 u
  4. (d) 111 u
Answer & solution

Answer: (d) 111 u

Answer: Option 4 – 111 u.
• CaCl₂ = 40 + 2×35.5 = 40 + 71 =111 u.
RRB Group D 2025-26 – 11 August 2026 Shift 3 · Atomic mass and molecular mass

Q20. Molecules of compounds are formed by:

  1. (a) Different elements in fixed ratios
  2. (b) Physical mixing of atoms
  3. (c) Similar atoms only
  4. (d) Random atomic combinations
Answer & solution

Answer: (a) Different elements in fixed ratios

Answer: Option 1 – Different elements in fixed ratios.
• A compound's molecules are formed bydifferent elements combined in fixed ratios.
RRB Group D 2025-26 – 10 August 2026 Shift 1 · Laws of chemical combination

Q21. In a chemical reaction, 12g of carbon reacts completely with 32g of oxygen to form carbon dioxide. If only 40g of carbon dioxide is collected, which of the following is the correct conclusion based on the Law of Conservation of Mass?

  1. (a) The reaction is incomplete because 4g of mass is unaccounted for.
  2. (b) Mass is always lost as energy, so the final mass decreases.
  3. (c) The law of conservation of mass does not apply to gas formation.
  4. (d) Carbon dioxide gas escaped during the reaction, so mass was not fully collected.
Answer & solution

Answer: (d) Carbon dioxide gas escaped during the reaction, so mass was not fully collected.

Answer: Option 4 — CO₂ gas escaped during the reaction, so mass was not fully collected
12 + 32 = 44 g should form; only 40 g collected because someCO₂ gas escaped(mass is still conserved) → Option 4.
RRB Group D 2025-26 – 9 August 2026 Shift 1 · Atomic mass and molecular mass

Q22. Which statement correctly explains the term atomicity with reference to molecules of elements?

  1. (a) It is the number of electrons in an element
  2. (b) It is the total mass of atoms present in one molecule
  3. (c) It is the number of shells present in an atom
  4. (d) It is the number of atoms present in one molecule
Answer & solution

Answer: (d) It is the number of atoms present in one molecule

Answer: Option 4 — Number of atoms present in one molecule
Atomicity =number of atoms in one moleculeof an element → Option 4.
RRB Group D 2025-26 – 9 August 2026 Shift 2 · Atomic mass and molecular mass

Q23. Which of the following does not exist as a diatomic molecule?

  1. (a) Sulphur
  2. (b) Nitrogen
  3. (c) Chlorine
  4. (d) Hydrogen
Answer & solution

Answer: (a) Sulphur

Answer: Option 1 — Sulphur
Nitrogen, chlorine and hydrogen are diatomic;sulphurexists as S₈, not diatomic → Option 1.
RRB Group D 2025-26 – 9 August 2026 Shift 2 · Atomic mass and molecular mass

Q24. A student is asked to calculate the formula unit mass of barium nitrate (Ba(NO₃)₂). What should be the student's first step ?

  1. (a) Find the elements in the formula.
  2. (b) Find the molecular mass of nitrate only
  3. (c) Find the atomic mass of barium only
  4. (d) Find the atomic number of barium
Answer & solution

Answer: (a) Find the elements in the formula.

Answer: Option 1 — Find the elements in the formula
The first step to find formula unit mass is toidentify the elements present→ Option 1.
RRB Group D 2025-26 – 9 August 2026 Shift 3 · Valency and chemical formulae

Q25. A student wants to write the chemical formula for aluminium sulfate, which contains two aluminium atoms, three sulfur atoms, and twelve oxygen atoms. Which is the correct formula?

  1. (a) Al₃(SO₄)₂
  2. (b) Al₂SO₄
  3. (c) Al(SO₄)₃
  4. (d) Al₂(SO₄)₃
Answer & solution

Answer: (d) Al₂(SO₄)₃

Answer: Option 4 — Al₂(SO₄)₃
Aluminium sulfate isAl₂(SO₄)₃(2 Al, 3 S, 12 O) → Option 4.
RRB Group D 2025-26 – 6 August 2026 Shift 2 · Laws of chemical combination

Q26. When 16 g of oxygen is combined with 2 g of hydrogen, 18 g water is formed. This observation is in accordance with:

  1. (a) Gay-Lussac’s law
  2. (b) law of multiple proportions
  3. (c) Avagadro’s Law
  4. (d) law of conservation of mass
Answer & solution

Answer: (d) law of conservation of mass

Answer: Option 4 — Law of conservation of mass
16 g + 2 g = 18 g (mass conserved) — this illustrates thelaw of conservation of mass→ Option 4.
RRB Group D 2025-26 – 6 August 2026 Shift 3 · Atomic mass and molecular mass

Q27. If the molecular mass of nitrogen gas (N2) is 28, what is the atomic mass of a single nitrogen atom?

  1. (a) 7
  2. (b) 14
  3. (c) 28
  4. (d) 21
Answer & solution

Answer: (b) 14

Answer: Option 2 — 14
N₂ molecular mass = 28 ⟹ atomic mass of one N atom = 28/2 =14→ Option 2.
RRB Group D 2025-26 – 5 August 2026 Shift 1 · Atomic mass and molecular mass

Q28. Which property is used to differentiate atoms of various elements based on the total mass of their protons and neutrons in the nucleus?

  1. (a) Isotopic ratio
  2. (b) Atomic number
  3. (c) Atomic mass
  4. (d) Valency
Answer & solution

Answer: (c) Atomic mass

Answer: Option 3 — Atomic mass
Atomic mass= total mass of protons + neutrons in the nucleus; it distinguishes atoms of different elements by mass → Option 3.
RRB Group D 2025-26 – 5 August 2026 Shift 1 · Laws of chemical combination

Q29. In ammonia (NH₃), the combining elements are present in the ratio of:

  1. (a) 7:3
  2. (b) 14:6
  3. (c) 14:3
  4. (d) 28:3
Answer & solution

Answer: (c) 14:3

Answer: Option 3 — 14:3
In NH₃: N = 14 u, H₃ = 3 × 1 = 3 u ⟹ mass ratio N : H =14 : 3→ Option 3.
RRB Group D 2025-26 – 5 August 2026 Shift 2 · Valency and chemical formulae

Q30. When potassium reacts with chlorine to form potassium chloride, which formula correctly represents the resulting compound based on their combining ratios?

  1. (a) KCl₂
  2. (b) K₂Cl₂
  3. (c) KCl
  4. (d) K₂Cl
Answer & solution

Answer: (c) KCl

Answer: Option 3 — KCl
K (valency 1) + Cl (valency 1) →KCl→ Option 3.
RRB Group D 2025-26 – 4 August 2026 Shift 1 · Ions and radicals

Q31. When a non-metal gains electrons, it forms:

  1. (a) Anion
  2. (b) Proton
  3. (c) Cation
  4. (d) Neutral
Answer & solution

Answer: (a) Anion

Answer: Option 1 — Anion
Gaining electrons gives anegative ion = anion→ Option 1.
RRB Group D 2025-26 – 4 August 2026 Shift 2 · Valency and chemical formulae

Q32. What does the valency of an element indicate?

  1. (a) The mass of the atom measured in atomic mass units
  2. (b) The total number of electron shells present in an atom
  3. (c) The total number of neutrons present in the nucleus
  4. (d) The combining capacity of an atom in chemical reactions
Answer & solution

Answer: (d) The combining capacity of an atom in chemical reactions

Answer: Option 4 — The combining capacity of an atom
Valency = thecombining capacityof an atom (number of electrons it gains/loses/shares) → Option 4.
RRB Group D 2025-26 – 4 August 2026 Shift 2 · Ions and radicals

Q33. An element X forms an ion X²⁺. If the neutral atom of X has 20 electrons, how many electrons are present in the ion?

  1. (a) 20
  2. (b) 22
  3. (c) 18
  4. (d) 24
Answer & solution

Answer: (c) 18

Answer: Option 3 — 18
X²⁺ means the atom lost 2 electrons: 20 − 2 =18 electrons→ Option 3.
RRB Group D 2025-26 – 4 August 2026 Shift 3 · Laws of chemical combination

Q34. Match the compound to its respective mass ratio by elements from the table given below.
[TABLE]
Compound | Mass Ratio by Elements
1. Water (H₂O) | A. 1 : 8
2. Carbon dioxide (CO₂) | B. 14 : 3
3. Ammonia (NH₃) | C. 3 : 8
4. Sulphur dioxide (SO₂) | D. 1 : 1
[/TABLE]

  1. (a) 1 → A, 2 → C, 3 → B, 4 → D
  2. (b) 1 → C, 2 → A, 3 → B, 4 → D
  3. (c) 1 → A, 2 → D, 3 → B, 4 → C
  4. (d) 1 → A, 2 → C, 3 → D, 4 → B
Answer & solution

Answer: (a) 1 → A, 2 → C, 3 → B, 4 → D

Answer: Option 1 — 1 → A, 2 → C, 3 → B, 4 → D
Matching each compound to its element mass-ratio gives1→A, 2→C, 3→B, 4→D→ Option 1. (Per the table given in the paper.)
RRB Group D 2025-26 – 4 August 2026 Shift 3 · Atomic mass and molecular mass

Q35. Which of the following best describes a molecule of a compound?

  1. (a) A molecule formed by the chemical combination of different elements in a fixed ratio
  2. (b) A physical mixture of atoms
  3. (c) A group of atoms of the same element
  4. (d) A molecule formed by atoms in any variable ratio
Answer & solution

Answer: (a) A molecule formed by the chemical combination of different elements in a fixed ratio

Answer: Option 1 — Formed by chemical combination of different elements in a fixed ratio
A compound's molecule = atoms of different elements combined chemically in afixed ratio→ Option 1.
RRB Group D 2025-26 – 3 August 2026 Shift 1 · Atomic mass and molecular mass

Q36. Calculate the formula unit masses of ZnO, Na₂O, if given atomic masses of Zn = 65 u, Na = 23 u, and O = 16 u.

  1. (a) ZnO = 48u and Na₂O = 62 u.
  2. (b) ZnO = 18u and Na₂O = 62 u.
  3. (c) ZnO = 81u and Na₂O = 62 u.
  4. (d) ZnO = 81u and Na₂O = 39 u.
Answer & solution

Answer: (c) ZnO = 81u and Na₂O = 62 u.

Answer: Option 3 — ZnO = 81 u and Na₂O = 62 u
Formula unit mass = sum of atomic masses.
• ZnO = 65 + 16 =81 u
• Na₂O = 2×23 + 16 = 46 + 16 =62 u
Both match Option 3.
RRB Group D 2025-26 – 3 August 2026 Shift 2 · Laws of chemical combination

Q37. A glass vessel contains a chemical reaction where no gas escapes or enters. According to the Law of Conservation of Mass, which situation is most accurate?

  1. (a) The total mass after the reaction increases.
  2. (b) The total mass after the reaction decreases.
  3. (c) The total mass before the reaction is less than that after the reaction.
  4. (d) The total mass before and after the reaction remains unchanged.
Answer & solution

Answer: (d) The total mass before and after the reaction remains unchanged.

Answer: Option 4 — Total mass before and after remains unchanged
Law of Conservation of Mass:in a closed system (no gas in/out), mass is neither created nor destroyed, so mass before = mass after → Option 4.
RRB Group D 2025-26 – 3 August 2026 Shift 3 · Atomic mass and molecular mass

Q38. Given that the atomic masses of C = 12 u, O = 16 u and H = 1 u, which of the following options correctly represents the molecular masses of CO₂ and H₂O?

  1. (a) CO₂ = 46 u ; H₂O = 20 u
  2. (b) CO₂ = 32 u ; H₂O = 18 u
  3. (c) CO₂ = 44 u ; H₂O = 18 u
  4. (d) CO₂ = 44 u ; H₂O = 16 u
Answer & solution

Answer: (c) CO₂ = 44 u ; H₂O = 18 u

Answer: Option 3 — CO₂ = 44 u ; H₂O = 18 u
CO₂ = 12 + 2(16) = 44 u; H₂O = 2(1) + 16 = 18 u → Option 3.
RRB NTPC UG CBT-1 2025-26 – 20 June 2026 Shift 1 · Mole concept and Avogadro number

Q39. Which information is essential to perform stoichiometric calculations for a chemical reaction?

  1. (a) A balanced chemical equation
  2. (b) The color of the reactants
  3. (c) Temperature of the reaction
  4. (d) Type of apparatus used
Answer & solution

Answer: (a) A balanced chemical equation

Stoichiometric Calculations
Stoichiometry deals with quantitative relationships between reactants and products in a chemical reaction.
Essential Requirement:
A balanced chemical equation is mandatory because:
• It shows the mole ratio of reactants to products
• Allows calculation of amounts of substances involved
• Follows Law of Conservation of Mass
Color, temperature, and apparatus type do not affect stoichiometric calculations.
Answer: Option 1 → A balanced chemical equation
RRB NTPC UG CBT-1 2025-26 – 17 June 2026 Shift 3 · Laws of chemical combination

Q40. Which of the following explains that during a chemical reaction, the sum of masses of the reactants and products remains unchanged?

  1. (a) Law of Periodicity
  2. (b) Law of Conservation of Mass
  3. (c) Law of Definite Composition
  4. (d) Law of Definite Proportions
Answer & solution

Answer: (b) Law of Conservation of Mass

Topic:Fundamental Laws of Chemistry
Law of Conservation of Mass(Lavoisier, 1789):
Mass can neither be created nor destroyed in a chemical reaction.
Total mass of reactants = Total mass of products
Other laws:
Law of Definite Proportions: Elements combine in fixed ratios by mass.
Law of Periodicity: Properties repeat periodically in periodic table.
Answer: Option 2 → Law of Conservation of Mass
RRB NTPC UG CBT-1 2025-26 – 14 June 2026 Shift 3 · Mole concept and Avogadro number

Q41. Which term refers to the quantitative study of reactants and products in a chemical reaction?

  1. (a) Mass per cent
  2. (b) Mole fraction
  3. (c) Stoichiometry
  4. (d) Molarity
Answer & solution

Answer: (c) Stoichiometry

Topic: Chemical Calculations
Stoichiometryis the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction.
It involves calculations based on balanced chemical equations to determine amounts of substances consumed and produced.
Other options are concentration terms:
• Mass per cent - concentration expression
• Mole fraction - ratio of moles
• Molarity - moles per liter
Answer: Option 3 → Stoichiometry
RRB NTPC UG CBT-1 2025-26 – 8 May 2026 Shift 3 · Valency and chemical formulae

Q42. Select the correct formula of Aluminium Sulphate.

  1. (a) Al3SO4
  2. (b) Al2SO4
  3. (c) Al(SO4)3
  4. (d) Al2(SO4)3
Answer & solution

Answer: (d) Al2(SO4)3

Aluminium Sulphate Formula
Aluminium (Al) has valency =+3
Sulphate (SO₄) has valency =-2
Cross-multiply valencies:
Al₂(SO₄)₃
This balances: 2×(+3) + 3×(-2) = +6 - 6 = 0
Answer: Option 4 → Al2(SO4)3
RRB NTPC GL CBT-1 2025-26 – 27 March 2026 Shift 1 · Atomic mass and molecular mass

Q43. What is the atomicity of a molecule of oxygen (O₂)?

  1. (a) 3
  2. (b) 2
  3. (c) 4
  4. (d) 1
Answer & solution

Answer: (b) 2

Atomicity= Number of atoms in one molecule
Oxygen molecule = O₂
Number of oxygen atoms in O₂ =2
Therefore, atomicity of O₂ = 2
Answer: Option 2 → 2
RRB NTPC GL CBT-1 2025-26 – 16 March 2026 Shift 1 · Dalton's atomic theory

Q44. Which postulate of Dalton's Atomic Theory was proven wrong by the discovery of subatomic particles?

  1. (a) Atoms of different elements have different masses.
  2. (b) Isotopes of the same element have different masses.
  3. (c) Matter consists of indivisible atoms.
  4. (d) Atoms retain their identity in chemical reactions.
Answer & solution

Answer: (c) Matter consists of indivisible atoms.

Dalton's Atomic Theory postulates
Dalton claimed atoms are indivisible and smallest units.
Discovery of electrons (1897), protons, neutrons proved atoms have subatomic particles.
Atoms can be divided into smaller components.
Other postulates remained valid: different elements have different atom masses, atoms combine in fixed ratios, atoms conserve identity in reactions.
Answer: Option 3 → Matter consists of indivisible atoms
RRB NTPC GL CBT-1 2025-26 – 16 March 2026 Shift 3 · Dalton's atomic theory

Q45. Which of the following scientists gave the concept of atomic mass?

  1. (a) J.J. Thomson
  2. (b) Niels Bohr
  3. (c) Ernest Rutherford
  4. (d) John Dalton
Answer & solution

Answer: (d) John Dalton

John Dalton proposed the concept of atomic mass in his Atomic Theory (1808).
Key points of Dalton's Atomic Theory:
• Matter is made of tiny, indivisible particles called atoms.
• Atoms of the same element are identical in mass and properties.
• Atoms of different elements differ in mass.
• He introduced the first atomic mass scale (relative to Hydrogen=1).
Why NOT others:
• J.J. Thomson → discovered electron (1897).
• Niels Bohr → proposed the planetary atomic model (1913).
• Ernest Rutherford → discovered the nucleus (1911).
Answer: Option 4 → John Dalton
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 3 · Ions and radicals

Q46. If an element forms an ion with a charge of −2, what does this indicate about its valency?

  1. (a) The element has 2 electrons in the nucleus.
  2. (b) The element has gained 2 electrons.
  3. (c) The element has gained 2 protons.
  4. (d) The element has lost 2 electrons.
Answer & solution

Answer: (b) The element has gained 2 electrons.

The charge of −2 on an ion means the element has gained 2 extra electrons.
Electrons have negative charge, so gaining electrons creates a negative ion.
Valency associated with negative charge shows electrons gained, not lost or protons changed.
Therefore, the element has gained 2 electrons to form a −2 charge ion.
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 3 · Laws of chemical combination

Q47. In a chemical reaction, 5.6 g of sodium reacts with 8.0 g of chlorine to form sodium chloride. What is the mass of sodium chloride formed according to the Law of Conservation of Mass?

  1. (a) 8.0 g
  2. (b) 12.4 g
  3. (c) 14.4 g
  4. (d) 13.6 g
Answer & solution

Answer: (d) 13.6 g

According to the Law of Conservation of Mass,
mass of reactants = mass of products.
Mass of sodium = 5.6 g
Mass of chlorine = 8.0 g
Therefore, mass of sodium chloride = 5.6 g + 8.0 g = 13.6 g.
But option 3 is 14.4 g which is incorrect; correct sum is 13.6 g.
Hence, the correct answer is Option 4 with 13.6 g sodium chloride formed.
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 2 · Atomic mass and molecular mass

Q48. What is the formula unit mass of ammonium sulphate (NH₄)₂SO₄?

  1. (a) 132 u
  2. (b) 146 u
  3. (c) 98 u
  4. (d) 114 u
Answer & solution

Answer: (a) 132 u

Ammonium sulphate formula: (NH4)2SO4
Calculate atomic masses: N = 14 u, H = 1 u, S = 32 u, O = 16 u
Mass of two NH4 groups: 2 × (14 + 4×1) = 2 × 18 = 36 u
Mass of SO4 group: 32 + 4×16 = 32 + 64 = 96 u
Total formula unit mass = 36 + 96 = 132 u
Check calculation: (NH4)2 = 36 u, SO4 = 96 u, Sum = 132 u
Option 1 states 132 u, but usually ammonium sulphate's molar mass is around 132 g/mol
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 1 · Atomic mass and molecular mass

Q49. Which of the following correctly represents the unit mass of a molecule of water (H₂O)?

  1. (a) 20 u
  2. (b) 10 u
  3. (c) 18 u
  4. (d) 16 u
Answer & solution

Answer: (c) 18 u

Molecular mass of water (H2O) is the sum of atomic masses of 2 hydrogen atoms and 1 oxygen atom.
Atomic mass of hydrogen (H) = 1 u.
Atomic mass of oxygen (O) = 16 u.
Total molecular mass = (2 × 1) + 16 = 18 u.
Hence, the unit mass of a molecule of water is 18 u.
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 1 · Valency and chemical formulae

Q50. Which of the following pairs of compounds contain the same total number of atoms per molecule?

  1. (a) C₂H₆O and PCl₃
  2. (b) NH₃ and CH₄
  3. (c) H₂SO₄ and N₂O₅
  4. (d) CO₂ and H₂O₂
Answer & solution

Answer: (c) H₂SO₄ and N₂O₅

Count total atoms in CO₂: C (1) + O (2) = 3 atoms
Count total atoms in H₂O₂: H (2) + O (2) = 4 atoms
So options 4 total atoms are 3 and 4, not equal
Check others quickly:
Option 1: C₂H₆O (2+6+1=9), PCl₃ (1+3=4), not equal
Option 2: NH₃ (1+3=4), CH₄ (1+4=5), not equal
Option 3: H₂SO₄ (2+1+4=7), N₂O₅ (2+5=7), equal atoms
Therefore, Option 3 compounds have the same total number of atoms per molecule.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Laws of chemical combination

Q51. A student heats 10 g of calcium carbonate in a closed container. It decomposes into 5.6 g of calcium oxide and carbon dioxide gas. What will be the mass of carbon dioxide produced?

  1. (a) 10.0 g
  2. (b) 5.0 g
  3. (c) 4.4 g
  4. (d) 15.6 g
Answer & solution

Answer: (c) 4.4 g

The reaction is: Calcium carbonate (CaCO3) → Calcium oxide (CaO) + Carbon dioxide (CO2).
Mass of calcium carbonate = 10 g
Mass of calcium oxide = 5.6 g
By conservation of mass: Total mass before = total mass after
Mass of CO2 = Mass of CaCO3 − Mass of CaO
Mass of CO2 = 10 g − 5.6 g = 4.4 g
Therefore, the mass of carbon dioxide produced is 4.4 g.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 1 · Atomic mass and molecular mass

Q52. Read the following three statements and choose the correct option:
Statement I: Formula unit mass is applicable only to ionic compounds, not covalent ones.
Statement II: Formula unit mass is the sum of the atomic masses of all atoms in the empirical formula of an ionic compound.
Statement III: The formula unit mass of sodium chloride (NaCl) is 58.5 u.

  1. (a) Only II and III are correct.
  2. (b) Only I and III are correct.
  3. (c) Only I and II are correct.
  4. (d) All I, II and III are correct.
Answer & solution

Answer: (d) All I, II and III are correct.

Statement I:Formula unit mass is used forionic compounds only(like NaCl), while covalent compounds usemolecular mass→ Correct.
Statement II:It is defined as thesum of atomic masses of atoms in the empirical formula→ Correct.
Statement III:NaCl = 23 (Na) + 35.5 (Cl) =58.5 u→ Correct.
Hence,all three statements are correct.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 3 · Numericals on mole concept

Q53. The molecular mass of a compound is '60'. If it contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen, what is the molecular formula?

  1. (a) C₃H₆O₃
  2. (b) CH₂O
  3. (c) C₄H₈O₄
  4. (d) C₂H₄O₂
Answer & solution

Answer: (d) C₂H₄O₂

Solution:
Assume 100 g compound ⇒ C = 40 g, H = 6.7 g, O = 53.3 g
Moles: C = 40/12 ≈ 3.33, H = 6.7/1 ≈ 6.7, O = 53.3/16 ≈ 3.33
Divide by smallest (3.33) ⇒ C : H : O = 1 : 2 : 1 ⇒ empirical formula =CH₂O
Empirical mass = 12 + 2 + 16 = 30
Molecular mass = 60 ⇒ factor = 60/30 = 2
Molecular formula = (CH₂O)₂ =C₂H₄O₂
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Atomic mass and molecular mass

Q54. The ratio of the unit mass of a molecule of nitrogen (N₂) to that of an oxygen molecule (O₂) is approximately:

  1. (a) 8 : 7
  2. (b) 7 : 8
  3. (c) 28 : 16
  4. (d) 14 : 32
Answer & solution

Answer: (b) 7 : 8

Molecular mass:
N₂ = 14 × 2 = 28
O₂ = 16 × 2 = 32
Ratio (N₂ : O₂) = 28 : 32
Simplify → 7 : 8
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 1 · Laws of chemical combination

Q55. Which of the following compounds obeys the Law of Constant Proportions?

  1. (a) Air
  2. (b) Water
  3. (c) Soil
  4. (d) Brass
Answer & solution

Answer: (b) Water

Correct option: 2
The Law of Constant Proportions states that a chemical compound always contains the same elements in the same fixed ratio by mass, regardless of the source or preparation method.
Among the options:
- Air is a mixture, not a compound, so it does not obey the law.
- Water (H2O) is a compound with a fixed ratio of hydrogen to oxygen by mass (approximately 1:8).
- Soil is a mixture of various substances, varies in composition.
- Brass is an alloy, a mixture of copper and zinc in varying proportions.
Therefore, water obeys the Law of Constant Proportions.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Valency and chemical formulae

Q56. Which of the following is the correct formula for the compound calcium phosphate?

  1. (a) CaPO₄
  2. (b) Ca₂(PO₄)₃
  3. (c) Ca₃(PO₄)₂
  4. (d) Ca₄(PO₄)₃
Answer & solution

Answer: (c) Ca₃(PO₄)₂

The compound calcium phosphate is composed of calcium ions Ca2+ and phosphate ions PO₄3-.
To balance the charges, 3 calcium ions (total charge +6) combine with 2 phosphate ions (total charge -6) to make a neutral compound.
Thus, the correct formula is Ca₃(PO₄)₂.
Hence, the correct option is 3.
Answer closed.
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 1 · Laws of chemical combination

Q57. If 9 g of water contains 1 g of hydrogen and 8 g of oxygen, then 36 g of water will contain:

  1. (a) 4 g of hydrogen and 24 g of oxygen
  2. (b) 3 g of hydrogen and 24 g of oxygen
  3. (c) 3 g of hydrogen and 32 g of oxygen
  4. (d) 4 g of hydrogen and 32 g of oxygen
Answer & solution

Answer: (d) 4 g of hydrogen and 32 g of oxygen

Water (H2O) consists of hydrogen and oxygen.
9 g of water contains 1 g hydrogen and 8 g oxygen.
To find hydrogen and oxygen in 36 g of water, multiply quantities by 4 (since 36/9 = 4).
Hydrogen = 1 g × 4 = 4 g.
Oxygen = 8 g × 4 = 32 g.
But the total is 4 + 32 = 36 g which fits water composition perfectly.
Wait, the option with 4 g hydrogen and 24 g oxygen matches? Recalculate oxygen carefully:
Oxygen in 9 g water = 8 g.
So, oxygen in 36 g water = 8 g × 4 = 32 g.
Thus, 36 g water contains 4 g hydrogen and 32 g oxygen.
Correct option is 4, but given options, 4 g hydrogen and 24 g oxygen is option 1, and 4 g hydrogen and 32 g oxygen is option 4.
Final answer: Option 4
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 3 · Atomic mass and molecular mass

Q58. What is the formula unit mass of sodium carbonate (Na₂CO₃)?
(Atomic masses: Na = 23 u, C = 12 u, O = 16 u)

  1. (a) 62 u
  2. (b) 98 u
  3. (c) 106 u
  4. (d) 84 u
Answer & solution

Answer: (c) 106 u

The formula unit mass is the sum of the atomic masses of all atoms in the formula.
Na₂CO₃ consists of 2 sodium (Na), 1 carbon (C), and 3 oxygen (O) atoms.
Calculate as: (2 × 23) + 12 + (3 × 16) = 46 + 12 + 48 = 106 u.
Re-checking the calculation: 46 + 12 + 48 = 106 u, which corresponds to Option 3.
Hence the formula unit mass is 106 u.
Final answer: Option 2
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 1 · Atomic mass and molecular mass

Q59. Which calculation correctly determines the formula unit mass of CaCl₂?

  1. (a) Atomic mass of Ca + (2 × atomic mass of Cl) = 40 + 71 = 111 u
  2. (b) Atomic mass of Ca + (atomic mass of Cl ÷ 2) = 40 + 17.75 = 57.75 u
  3. (c) Atomic mass of Ca + atomic mass of Cl = 40 + 35.5 = 75.5 u
  4. (d) Atomic mass of Ca × atomic mass of Cl = 40 × 35.5 = 1420 u
Answer & solution

Answer: (a) Atomic mass of Ca + (2 × atomic mass of Cl) = 40 + 71 = 111 u

The formula unit mass of CaCl₂ is calculated by adding the atomic mass of one calcium atom to the atomic mass of two chlorine atoms.
Atomic mass of Ca is 40 u.
Atomic mass of Cl is 35.5 u; for two Cl atoms it is 2 × 35.5 = 71 u.
Sum = 40 + 71 = 111 u.
Therefore, option 1 is the correct calculation for formula unit mass of CaCl₂.
Final answer: Option 1
RRB JE CBT-1 2025-26 – 25 February 2026 Shift 3 · Laws of chemical combination

Q60. Which observation supports the law of constant proportions for water?

  1. (a) Hydrogen and oxygen ratio is fixed
  2. (b) Water exists in three states
  3. (c) Water conducts electricity weakly
  4. (d) Water boils at fixed temperature
Answer & solution

Answer: (a) Hydrogen and oxygen ratio is fixed

The law of constant proportions states that a chemical compound always contains the same elements in the same fixed ratio by mass.
For water, this means the ratio of hydrogen to oxygen is always constant.
Option 1 says 'Hydrogen and oxygen ratio is fixed', which directly supports this law.
Other options describe physical properties, not composition ratios.
Hence, option 1 is correct.
Final answer: Option 1
RRB JE CBT-1 2025-26 – 25 February 2026 Shift 3 · Valency and chemical formulae

Q61. Why are brackets used in the formula Ca(OH)₂?

  1. (a) To show ionic nature
  2. (b) To balance oxygen atoms
  3. (c) To represent molecular bonding
  4. (d) To indicate two hydroxide groups
Answer & solution

Answer: (d) To indicate two hydroxide groups

The brackets in the formula Ca(OH)₂ are used to show that the hydroxide group (OH) is present twice.
It indicates two hydroxide ions are bonded with one calcium ion.
Without brackets, it would be unclear how many OH groups are attached.
Hence, brackets clarify the composition of the compound.
Correct option is 4.
This completes the answer.
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 3 · Valency and chemical formulae

Q62. Refer to the two statements given below and choose the CORRECT option.Statement A: While writing the chemical formula of a compound, the total positive and negative charges must balance each other.Statement B: Balancing valencies ensures that the compound formed is electrically neutral.

  1. (a) Statement A is correct but B is incorrect.
  2. (b) Statement A is incorrect but B is correct.
  3. (c) Both statements A and B are correct.
  4. (d) Both statements A and B are incorrect.
Answer & solution

Answer: (c) Both statements A and B are correct.

Both statements A and B talk about electrical charge balance in chemical compounds.
Statement A is correct: Chemical formula writing requires total positive and negative charges to balance.
Statement B is also correct: Balancing valencies ensures the compound formed is electrically neutral.
Hence, both statements are true and related concepts in chemistry.
Final answer: Option 3
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 2 · Laws of chemical combination

Q63. According to the law of constant proportions, the compound in which hydrogen and oxygen are present in a fixed mass ratio of 1:8 is:

  1. (a) Hydrogen peroxide
  2. (b) Water
  3. (c) Methane
  4. (d) Ammonia
Answer & solution

Answer: (b) Water

The law of constant proportions states that a chemical compound always contains the same elements in the exact same fixed ratio by mass.
Water (H2O) contains hydrogen and oxygen in a fixed mass ratio of 1:8.
Other compounds like hydrogen peroxide have different ratios.
Therefore, the compound with hydrogen and oxygen in a fixed mass ratio of 1:8 is water.
Hence, the correct answer is Option 2.
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 1 · Atomic mass and molecular mass

Q64. Which of the following compounds has a formula unit mass of 100 u?

  1. (a) MgO
  2. (b) Na2CO3
  3. (c) CaCl2
  4. (d) CaCO3
Answer & solution

Answer: (d) CaCO3

We calculate formula unit mass by adding atomic masses of each atom in the formula.
Option 1: MgO = Mg (24) + O (16) = 40 u.
Option 2: Na2CO3 = 2×Na (23) + C (12) + 3×O (16) = 46 + 12 + 48 = 106 u.
Option 3: CaCl2 = Ca (40) + 2×Cl (35.5) = 40 + 71 = 111 u.
Option 4: CaCO3 = Ca (40) + C (12) + 3×O (16) = 40 + 12 + 48 = 100 u.
Therefore, CaCO3 has a formula unit mass of 100 u.
Final answer: Option 4
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 1 · Atomic mass and molecular mass

Q65. Atomic masses of elements are compared with a standard reference. Carbon-12 was chosen as the standard for the atomic mass unit mainly because:

  1. (a) Carbon is able to form a large number of compounds
  2. (b) Carbon-12 gives atomic masses close to whole numbers
  3. (c) Carbon shows very little chemical reactivity with elements
  4. (d) Carbon is present in very large amounts in nature
Answer & solution

Answer: (b) Carbon-12 gives atomic masses close to whole numbers

Correct option: 2
Carbon-12 is used as the standard for atomic mass units because its atomic mass is exactly 12 units.
This choice makes all atomic masses close to whole numbers, simplifying calculations and comparisons.
It is a defined standard agreed upon internationally.
Thus, Carbon-12 provides a precise and convenient reference point for atomic masses.
Answer complete.
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 2 · Ions and radicals

Q66. If an atom loses two electrons, which of the following changes will occur?

  1. (a) The atomic number will increase by 2.
  2. (b) The atomic number will decrease by 2.
  3. (c) The atom will become a cation with +2 charge.
  4. (d) The neutron number will decrease by 2.
Answer & solution

Answer: (c) The atom will become a cation with +2 charge.

When an atom loses electrons, the atomic number does not change because it is the number of protons that defines it.
Losing two electrons makes the atom positively charged by +2, forming a cation with a +2 charge.
Neutrons are unaffected by loss of electrons.
Therefore, the correct change is that the atom becomes a cation with +2 charge.
Final answer: Option 3
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 1 · Ions and radicals

Q67. Sodium hydroxide (NaOH) contains __________ as its anion.

  1. (a) OH-
  2. (b) H+
  3. (c) Na+
  4. (d) NO₃⁻
Answer & solution

Answer: (a) OH-

Sodium hydroxide (NaOH) dissociates in water to give sodium ions (Na+) and hydroxide ions (OH-).
The anion present in NaOH is the hydroxide ion (OH-).
Other options: H+ is a cation, Na+ is a cation, NO₃⁻ is the nitrate ion, not part of NaOH.
Hence, the correct anion in sodium hydroxide is OH-.
Final answer: Option 1
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 1 · Laws of chemical combination

Q68. When two aqueous solutions react inside a sealed flask and form products, what happens to the total mass of the system?

  1. (a) It fluctuates during reaction
  2. (b) It increases due to gas formation
  3. (c) It decreases due to precipitation
  4. (d) It remains unchanged throughout
Answer & solution

Answer: (d) It remains unchanged throughout

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
In a sealed flask, no mass can enter or leave the system.
Therefore, the total mass of the reactants and products remains constant throughout the reaction.
Any physical changes like gas formation or precipitation do not alter the total mass inside the sealed container.
Hence, total mass remains unchanged during the reaction.
Final answer: Option 4
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 2 · Atomic mass and molecular mass

Q69. Which calculation correctly gives the molecular mass of nitric acid (HNO₃)?

  1. (a) 1 + 14 + 48
  2. (b) 1 + 14 + 16
  3. (c) 1 + 14 + 3
  4. (d) 2 + 14 + 48
Answer & solution

Answer: (a) 1 + 14 + 48

The molecular formula of nitric acid is HNO₃.
Atomic masses: H = 1, N = 14, O = 16.
Molecular mass = mass of H + mass of N + 3 × mass of O
= 1 + 14 + 3×16
= 1 + 14 + 48 = 63
Option 1 correctly sums the masses as 1 + 14 + 48.
Thus, the correct molecular mass calculation is Option 1.
Final answer: Option 1
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 2 · Ions and radicals

Q70. Which statement correctly describes an ion?

  1. (a) A molecule containing similar atoms only
  2. (b) A compound formed by covalent bonds
  3. (c) A charged atom or group of atoms
  4. (d) A neutral atom formed by chemical bonding
Answer & solution

Answer: (c) A charged atom or group of atoms

Option 3 is correct.
An ion is an atom or group of atoms that carries an electric charge due to loss or gain of electrons.
This makes it charged, unlike neutral atoms or molecules.
Options 1, 2, and 4 describe molecules or compounds, not ions.
Hence, a charged atom or group of atoms correctly defines an ion.
Answer completed.
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 2 · Atomic mass and molecular mass

Q71. Which substance has a molecular mass of 44 u?

  1. (a) Carbon dioxide
  2. (b) Methane
  3. (c) Carbon monoxide
  4. (d) Ammonia
Answer & solution

Answer: (a) Carbon dioxide

Carbon dioxide (CO2) has a molecular mass of 44 u.
Calculate molecular mass: C = 12 u, O = 16 u each.
So, 12 + 16 × 2 = 44 u.
Hence, carbon dioxide matches the molecular mass 44 u.
Final answer: Option 1
SSC MTS 2025 – 11 February 2026 Shift 1 · Dalton's atomic theory

Q72. Which of the following is a postulate of Dalton's atomic theory?
I. All matter is made of very tiny particles called atoms, which participate in chemical reactions.
II. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

  1. (a) Only I
  2. (b) Only II
  3. (c) Both I and II
  4. (d) Neither I nor II
Answer & solution

Answer: (c) Both I and II

The correct answer is Both I and II (Option 3). Both statements are postulates of Dalton's Atomic Theory (1808): Statement I — All matter is composed of extremely small, indivisible particles called atoms that participate in chemical reactions. Statement II — Atoms cannot be created or destroyed in a chemical reaction (law of conservation of mass). Both are fundamental postulates, so Options 1, 2, and 4 are incomplete or incorrect.
SSC MTS 2025 – 9 February 2026 Shift 1 · Laws of chemical combination

Q73. In a compound like water, the mass ratio of hydrogen to oxygen is always _____.

  1. (a) 1:14
  2. (b) 1:8
  3. (c) 1:2
  4. (d) 8:1
Answer & solution

Answer: (b) 1:8

Correct Answer: 2 (1:8)
Water (H2O) always has a fixed mass ratio of Hydrogen to Oxygen:
- H: 2 atoms × atomic mass 1 = 2 g
- O: 1 atom × atomic mass 16 = 16 g
- Ratio H:O = 2:16 = 1:8
This is in accordance with the Law of Definite Proportions (Law of Constant Composition), which states that a chemical compound always contains its component elements in fixed ratio by mass.
Option 1 (1:14) is incorrect.
Option 3 (1:2) is incorrect - this would be the ratio by number of atoms, not mass.
Option 4 (8:1) is incorrect - this is the reverse ratio (O:H).
SSC MTS 2025 – 6 February 2026 Shift 2 · Atomic mass and molecular mass

Q74. What is molecular mass?

  1. (a) The total number of atoms in a molecule
  2. (b) The sum of the valencies of the atoms in a molecule
  3. (c) The sum of the atomic masses of all atoms in a molecule
  4. (d) The volume occupied by a molecule
Answer & solution

Answer: (c) The sum of the atomic masses of all atoms in a molecule

Correct Answer: 3 (The sum of the atomic masses of all atoms in a molecule)
Molecular mass (also called molecular weight) is defined as the sum of the atomic masses of all the atoms present in one molecule of a substance. It is expressed in atomic mass units (amu) or Daltons (Da). For example, the molecular mass of water (H2O) = 2(1) + 16 = 18 amu.
Option 1 (total number of atoms) is incorrect - that is simply the atom count, not the mass.
Option 2 (sum of valencies) is incorrect - valency is the combining capacity of atoms, not their mass.
Option 4 (volume occupied) is incorrect - that refers to molar volume, not molecular mass.
RRB NTPC UG CBT-1 2024-25 – 9 September 2025 Shift 3 · Dalton's atomic theory

Q75. When did John Dalton publish his atomic theory that holds that all elements are made up of tiny, indivisible particles?

  1. (a) 1803
  2. (b) 1798
  3. (c) 1800
  4. (d) 1808
Answer & solution

Answer: (d) 1808

RRB NTPC UG CBT-1 2024-25 – 3 September 2025 Shift 1 · Laws of chemical combination

Q76. Which law states that a compound always contains the same elements in the same proportion by mass?

  1. (a) Law of Conservation of Mass
  2. (b) Law of Definite Proportions
  3. (c) Law of Multiple Proportions
  4. (d) Avogadro’s Law
Answer & solution

Answer: (b) Law of Definite Proportions

RRB NTPC UG CBT-1 2024-25 – 29 August 2025 Shift 2 · Dalton's atomic theory

Q77. Which postulate of Dalton’s atomic theory explains the Law of Definite Proportions?

  1. (a) Atoms of the same element are identical in mass and size.
  2. (b) Atoms are indivisible and indestructible.
  3. (c) Atoms of different elements have different masses and properties.
  4. (d) Atoms combine in simple whole-number ratios to form compounds.
Answer & solution

Answer: (d) Atoms combine in simple whole-number ratios to form compounds.

RRB NTPC UG CBT-1 2024-25 – 21 August 2025 Shift 3 · Laws of chemical combination

Q78. Which fundamental law of chemistry is illustrated by the fact that hydrogen and oxygen always combine in a fixed 1 : 8 mass ratio to form water?

  1. (a) Dalton's Atomic Theory
  2. (b) Law of Conservation of Energy
  3. (c) Law of Multiple Proportions
  4. (d) Law of Constant Proportions
Answer & solution

Answer: (d) Law of Constant Proportions

RRB NTPC UG CBT-1 2024-25 – 12 August 2025 Shift 3 · Dalton's atomic theory

Q79. Which postulate of Dalton’s atomic theory reflects the Law of Conservation of Mass?

  1. (a) Atoms of different elements have different masses and properties.
  2. (b) Atoms combine in simple whole-number ratios to form compounds.
  3. (c) Atoms cannot be created or destroyed in a chemical reaction.
  4. (d) All matter is made up of indivisible particles called atoms.
Answer & solution

Answer: (c) Atoms cannot be created or destroyed in a chemical reaction.

RRB NTPC GL CBT-1 2024-25 – 14 June 2025 Shift 2 · Laws of chemical combination

Q80. Which pair of compounds illustrates the Law of Multiple Proportions?

  1. (a) CaCO₃ and CaCl₂
  2. (b) NaOH and NaCl
  3. (c) HCl and HBr
  4. (d) CO and CO₂
Answer & solution

Answer: (d) CO and CO₂

RRB NTPC GL CBT-1 2024-25 – 11 June 2025 Shift 1 · Numericals on mole concept

Q81. In the reaction: 2H₂ + O₂ → 2H₂O, if 5 moles of H₂ and 2 moles of O₂ are mixed, the limiting reagent is:

  1. (a) H₂O
  2. (b) H₂
  3. (c) CO₂
  4. (d) O₂
Answer & solution

Answer: (d) O₂

RRB NTPC GL CBT-1 2024-25 – 10 June 2025 Shift 2 · Numericals on mole concept

Q82. A compound has an empirical formula CH₂ and a molar mass of 56 g/mol. Its molecular formula is:

  1. (a) C₄H₈
  2. (b) CH₂
  3. (c) C₃H₆
  4. (d) C₂H₄
Answer & solution

Answer: (a) C₄H₈

RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Ions and radicals

Q83. Which of the following elements is most likely to form a negative ion (anion)?

  1. (a) Sodium (Na)
  2. (b) Chlorine (Cl)
  3. (c) Aluminium (Al)
  4. (d) Calcium (Ca)
Answer & solution

Answer: (b) Chlorine (Cl)

Correct Answer: Option 2 — Chlorine (Cl)
An anion is a negatively charged ion, formed when an atom gains electrons. Atoms do this to reach the stable octet of the nearest noble gas. It is the non-metals, which have 5, 6 or 7 electrons in their outermost shell, that find it easier to gain a few electrons than to lose many — so non-metals form anions, while metals form cations by losing electrons.
Chlorine (Z = 17) has the electronic configuration 2, 8, 7. It needs just one electron to complete its octet, so it readily gains one and becomes the chloride ion, Cl⁻ with the configuration 2, 8, 8 — the same as argon. Chlorine is therefore the only non-metal in the list and the obvious answer.
⚗️ The Reaction
Cl + e⁻ → Cl⁻ (2, 8, 7 → 2, 8, 8)
Na → Na⁺ + e⁻ (2, 8, 1 → 2, 8)
• In NaCl, sodium donates its single valence electron and chlorine accepts it, giving Na⁺ and Cl⁻ held by an ionic (electrovalent) bond.
• Non-metals also have high electronegativity and high electron affinity, which is why they attract electrons.
❌ Why the other options are incorrect
✗ Sodium (Na) — a metal with configuration 2, 8, 1; it loses 1 electron to form the cation Na⁺, not an anion.
✗ Aluminium (Al) — a metal with configuration 2, 8, 3; it loses 3 electrons to form Al³⁺, a positive ion.
✗ Calcium (Ca) — a metal with configuration 2, 8, 8, 2; it loses 2 electrons to form Ca²⁺, again a cation.
📌 SSC & Railway Exam Facts
• Cation = positive ion (loses electrons), moves to the cathode; anion = negative ion (gains electrons), moves to the anode.
• Metals are electropositive and form cations; non-metals are electronegative and form anions.
• Fluorine is the most electronegative element; caesium is the most electropositive.
• Group 17 (halogens) gain 1 electron → valency 1; group 16 gains 2 → valency 2.
• Ionic compounds have high melting points, are soluble in water and conduct electricity in molten or aqueous state, not in the solid state.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 2 · Atomic mass and molecular mass

Q84. Calculate the molecular mass of ammonia, NH₃, using the provided atomic masses (Atomic mass of N = 14 u and H = 1 u).

  1. (a) 42 u
  2. (b) 17 u
  3. (c) 21 u
  4. (d) 10 u
Answer & solution

Answer: (b) 17 u

Correct Answer: Option 2 — 17 u
The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of it. Its unit is the unified atomic mass unit, u.
The formula NH₃ tells us that one molecule contains 1 atom of nitrogen and 3 atoms of hydrogen — the subscript 3 applies only to the H written just before it.
⚗️ The Calculation
Molecular mass of NH₃ = (1 × mass of N) + (3 × mass of H)
= (1 × 14) + (3 × 1)
= 14 + 3 = 17 u
• Do not multiply 14 by 3 — the subscript belongs to hydrogen only, not to nitrogen.
• Since 1 mole = molecular mass in grams, 1 mole of NH₃ = 17 g.
Ammonia is a colourless gas with a pungent smell, highly soluble in water, and is made industrially by the Haber process from N₂ and H₂. Its aqueous solution is basic, turning red litmus blue.
❌ Why the other options are incorrect
✗ 42 u — obtained by wrongly taking 3 × (14) = 42, i.e. applying the subscript 3 to nitrogen instead of hydrogen.
✗ 21 u — obtained by wrongly adding 14 + 7 or by treating hydrogen's mass as more than 1 u; the hydrogen contribution is only 3 u.
✗ 10 u — obtained by subtracting (14 − 3 − 1) instead of adding; molecular mass is always a sum, never a difference.
📌 SSC & Railway Exam Facts
• Molecular mass = sum of atomic masses of all atoms in the molecule; unit u.
• H₂O = 18 u, CO₂ = 44 u, NH₃ = 17 u, CH₄ = 16 u, H₂SO₄ = 98 u.
• 1 mole = 6.022 × 10²³ particles — Avogadro's number; its mass in grams equals the molecular mass.
• Ammonia (NH₃) is prepared by the Haber process: N₂ + 3H₂ ⇌ 2NH₃.
• Atomic mass unit (u) is defined as 1/12 of the mass of a carbon-12 atom.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Laws of chemical combination

Q85. Which of the following scientists proposed the 'Law of Constant Proportions'?

  1. (a) Joseph Proust
  2. (b) Ernest Rutherford
  3. (c) JJ Thomson
  4. (d) E Goldstein
Answer & solution

Answer: (a) Joseph Proust

Correct Answer: Option 1 — Joseph Proust
The Law of Constant Proportions, also called the Law of Definite Proportions, was given by the French chemist Joseph Louis Proust in 1799. It states that a pure chemical compound always contains the same elements combined in the same fixed proportion by mass, no matter where the sample comes from or how it is prepared.
For example, water (H₂O) always has hydrogen and oxygen in the mass ratio 1 : 8 — whether it is taken from a river, from rain or made in the laboratory. Likewise ammonia (NH₃) always has N : H = 14 : 3, and carbon dioxide (CO₂) always has C : O = 3 : 8.
This law, together with the Law of Conservation of Mass (Lavoisier), formed the experimental base on which John Dalton built his atomic theory in 1808.
❌ Why the other options are incorrect
✗ Ernest Rutherford — performed the α-particle scattering (gold foil) experiment and discovered the nucleus; he is called the Father of Nuclear Physics.
✗ JJ Thomson — discovered the electron (1897) and gave the plum-pudding (watermelon) model of the atom; he did not state any law of proportions.
✗ E Goldstein — discovered anode rays (canal rays), which led to the discovery of the proton; again unconnected with the law of constant proportions.
📌 SSC & Railway Exam Facts
• Law of Constant (Definite) Proportions — Joseph Proust; Law of Conservation of Mass — Antoine Lavoisier.
• In water, H : O = 1 : 8 by mass; in CO₂, C : O = 3 : 8; in NH₃, N : H = 14 : 3.
• Dalton's atomic theory (1808) explained both of these laws.
• Electron — J. J. Thomson, proton — E. Goldstein/Rutherford, neutron — James Chadwick (1932).
• Law of Multiple Proportions — John Dalton; Avogadro's law — Amedeo Avogadro.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Valency and chemical formulae

Q86. Which of the following elements does NOT usually form molecules and is found in its atomic form?

  1. (a) Helium
  2. (b) Nitrogen
  3. (c) Oxygen
  4. (d) Hydrogen
Answer & solution

Answer: (a) Helium

Correct Answer: Option 1 — Helium
Atomicity is the number of atoms present in one molecule of an element. Helium is a noble gas of group 18, and its atom already has a completely filled outermost shell — the electronic configuration of helium is simply 2, a complete duplet.
Because its outer shell is already full, helium has no tendency to gain, lose or share electrons. It therefore does not bond even with its own kind and exists as single, separate atoms. Such elements are called monatomic, and their atomicity is 1. All noble gases — He, Ne, Ar, Kr, Xe and Rn — are monatomic, which is why they are also called inert or noble gases.
By contrast, nitrogen, oxygen and hydrogen all have incomplete outer shells, so two of their atoms share electrons and exist as diatomic molecules with atomicity 2.
❌ Why the other options are incorrect
✗ Nitrogen — configuration 2, 5; two atoms share three pairs of electrons to form N₂ with a very strong triple bond; atomicity 2.
✗ Oxygen — configuration 2, 6; two atoms share two pairs to form O₂ with a double bond; atomicity 2. Its allotrope ozone, O₃, is triatomic.
✗ Hydrogen — configuration 1; two atoms share one pair to complete their duplets, forming H₂; atomicity 2.
📌 SSC & Railway Exam Facts
• Monatomic (atomicity 1): all noble gases — He, Ne, Ar, Kr, Xe, Rn.
• Diatomic (atomicity 2): H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
• Triatomic: O₃ (ozone); tetra-atomic: P₄; poly-atomic: S₈.
• Helium is the second most abundant element in the universe and is used in weather balloons because it is light and non-inflammable.
• Noble gases occupy group 18; helium has 2 electrons, the rest have 8 electrons in the outermost shell.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Valency and chemical formulae

Q87. The chemical formula of ammonium sulphate is ______.

  1. (a) Na₂SO₄
  2. (b) (NH₄)SO₄
  3. (c) (NH₄)₂SO₄
  4. (d) Ca₂SO₄
Answer & solution

Answer: (c) (NH₄)₂SO₄

Correct Answer: Option 3 — (NH₄)₂SO₄
A chemical formula is written by balancing the charges of the two radicals so that the compound as a whole is electrically neutral. The ammonium ion is NH₄⁺ (charge +1) and the sulphate ion is SO₄²⁻ (charge −2).
To cancel a charge of −2, we need two ammonium ions, each carrying +1. This is the criss-cross method: the valency of one ion becomes the subscript of the other. The two NH₄ units are enclosed in a bracket before writing the subscript 2, because NH₄ is a polyatomic (compound) radical — writing NH₄₂ would be wrong.
So ammonium sulphate = (NH₄)₂SO₄, containing 2 N, 8 H, 1 S and 4 O atoms and having a molecular mass of 132 u. It is a widely used nitrogenous fertiliser.
❌ Why the other options are incorrect
✗ Na₂SO₄ — this is sodium sulphate (Glauber's salt as the decahydrate); the cation is Na⁺, not ammonium.
✗ (NH₄)SO₄ — charge-wise unbalanced: +1 against −2; one ammonium ion cannot neutralise a sulphate ion.
✗ Ca₂SO₄ — wrong on two counts: the cation is calcium, and Ca²⁺ with SO₄²⁻ gives CaSO₄, never Ca₂SO₄.
📌 SSC & Railway Exam Facts
• Common radicals: NH₄⁺, Na⁺, Ca²⁺, Al³⁺, SO₄²⁻, CO₃²⁻, NO₃⁻, PO₄³⁻.
• In the criss-cross method the valency of one ion becomes the subscript of the other.
• A polyatomic radical taken more than once must be put in brackets: (NH₄)₂SO₄, Ca(OH)₂, Al₂(SO₄)₃.
• (NH₄)₂SO₄ and urea NH₂CONH₂ are nitrogenous fertilisers.
• CaSO₄·2H₂O = gypsum, CaSO₄·½H₂O = Plaster of Paris, Na₂SO₄·10H₂O = Glauber's salt.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Valency and chemical formulae

Q88. Which of the options below represents the correct chemical formula of calcium hydroxide?

  1. (a) KOH
  2. (b) Mg(OH)₂
  3. (c) NaOH
  4. (d) Ca(OH)₂
Answer & solution

Answer: (d) Ca(OH)₂

Correct Answer: Option 4 — Ca(OH)₂
A chemical formula is written by criss-crossing the valencies of the ions. Calcium forms the ion Ca²⁺ (valency 2) and the hydroxide radical is OH⁻ (valency 1). To make the compound electrically neutral, two OH⁻ ions are needed for every one Ca²⁺ ion, giving Ca(OH)₂. The bracket is essential — the subscript 2 outside it multiplies the whole OH group, so the formula stands for 1 Ca, 2 O and 2 H atoms.
Ca(OH)₂ is commonly called slaked lime or lime water. It is a strong base made by adding water to quick lime (CaO), an intensely exothermic reaction. Lime water is the standard laboratory test for carbon dioxide — it turns milky because insoluble CaCO₃ is formed. It is also used in whitewashing, in neutralising acidic soil and in making bleaching powder.
⚗️ The Reaction
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat
• Quick lime (CaO) plus water gives slaked lime, Ca(OH)₂ — a combination and strongly exothermic reaction.
• Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O — this is why lime water turns milky with CO₂.
❌ Why the other options are incorrect
✗ KOH — this is potassium hydroxide (caustic potash); K⁺ has a valency of 1, so only one OH⁻ is needed.
✗ Mg(OH)₂ — this is magnesium hydroxide, the milk of magnesia used as an antacid; the metal is magnesium, not calcium.
✗ NaOH — this is sodium hydroxide (caustic soda), obtained from the chlor-alkali process; Na⁺ also has a valency of 1.
📌 SSC & Railway Exam Facts
• Quick lime = CaO, slaked lime = Ca(OH)₂, limestone/marble/chalk = CaCO₃, gypsum = CaSO₄·2H₂O.
• Plaster of Paris = CaSO₄·½H₂O, made by heating gypsum to about 100 °C (373 K).
• Caustic soda = NaOH, caustic potash = KOH, milk of magnesia = Mg(OH)₂ (an antacid).
• Bleaching powder = CaOCl₂, made by passing Cl₂ over dry slaked lime.
• Bases turn red litmus blue, taste bitter, feel soapy, and have pH > 7.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Valency and chemical formulae

Q89. Which of the following represents the molecular formula for carbon monoxide?

  1. (a) CO
  2. (b) C₂O
  3. (c) CO₂
  4. (d) CO₃
Answer & solution

Answer: (a) CO

Correct Answer: Option 1 — CO
The prefix "mono" means one. Carbon monoxide therefore contains one carbon atom joined to one oxygen atom, and its molecular formula is simply CO. Reading the name is enough to write the formula: *mono*-oxide → one oxygen.
CO is formed by the incomplete combustion of carbon and carbon-containing fuels — burning coal, charcoal or petrol in a limited supply of oxygen. It is a colourless, odourless and highly poisonous gas. Breathing it is dangerous because CO binds to the haemoglobin of blood about 200–250 times more strongly than oxygen does, forming carboxyhaemoglobin and stopping oxygen from reaching the tissues. This is why a coal sigri must never be burnt in a closed room.
❌ Why the other options are incorrect
✗ C₂O — no such common oxide of carbon exists; the subscript 2 on carbon would mean two carbon atoms, which the name "monoxide" does not indicate.
✗ CO₂ — this is carbon dioxide, where "di" means two oxygen atoms; it is the product of complete combustion and of respiration.
✗ CO₃ — this is not a molecule but the carbonate radical, CO₃²⁻, which exists only inside compounds such as CaCO₃ and Na₂CO₃.
📌 SSC & Railway Exam Facts
• Prefixes: mono = 1, di = 2, tri = 3, tetra = 4 — CO, CO₂, SO₂, SO₃, CCl₄.
• CO is poisonous because it forms carboxyhaemoglobin with blood haemoglobin.
• Incomplete combustion → CO; complete combustion → CO₂.
• CO₂ turns lime water milky (forming CaCO₃) — the standard laboratory test for the gas.
• CO₂ is a greenhouse gas and is used in fire extinguishers; solid CO₂ is dry ice.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Atomic mass and molecular mass

Q90. The atomic mass of carbon-12 isotope is:

  1. (a) 14 u
  2. (b) 16 u
  3. (c) 12 u
  4. (d) 6 u
Answer & solution

Answer: (c) 12 u

Correct Answer: Option 3 — 12 u
The carbon-12 isotope (₆C¹²) has 6 protons and 6 neutrons, so its mass number is 6 + 6 = 12 and its atomic mass is exactly 12 u. This is not an approximation — C-12 is the modern standard of atomic mass itself.
Since 1961, the atomic mass unit (u), also written amu or dalton (Da), has been defined as exactly one-twelfth (1/12) of the mass of one atom of carbon-12. Every other atomic mass in the periodic table is quoted relative to this standard: an oxygen atom is 16 u because it is 16 times as heavy as 1/12 of a C-12 atom.
Note that the atomic mass of carbon printed in the periodic table is 12.011 u, because natural carbon is a mixture of the isotopes C-12 (about 98.9%) and C-13 (about 1.1%), with traces of C-14. The value 12 u exactly applies to the C-12 isotope alone.
❌ Why the other options are incorrect
✗ 14 u — this is the mass number of carbon-14, the radioactive isotope used in carbon dating; it is also the atomic mass of nitrogen.
✗ 16 u — this is the atomic mass of oxygen (₈O¹⁶), not of carbon.
✗ 6 u — 6 is the atomic number of carbon (its number of protons), not its atomic mass; this is the classic confusion between Z and A.
📌 SSC & Railway Exam Facts
• 1 u = 1/12 of the mass of one carbon-12 atom — C-12 is the standard for atomic masses.
• Carbon: Z = 6; isotopes C-12, C-13, C-14; the periodic-table value is 12.011 u.
• C-14 is radioactive with a half-life of about 5730 years and is used for carbon dating.
• Mass of proton ≈ mass of neutron ≈ 1 u; mass of electron ≈ 1/1836 u (negligible).
• Mass number (A) = protons + neutrons; atomic number (Z) = protons.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 1 · Laws of chemical combination

Q91. During a chemical reaction, 10 grams of substance A reacts with 15 grams of substance B to produce a new compound. According to the law of conservation of mass, what will be the total mass of the products?

  1. (a) 10 g
  2. (b) 11 g
  3. (c) 25 g
  4. (d) 16 g
Answer & solution

Answer: (c) 25 g

Correct Answer: Option 3 — 25 g
The law of conservation of mass, given by Antoine Lavoisier (1774), states that mass can neither be created nor destroyed in a chemical reaction. Therefore the total mass of the reactants is always equal to the total mass of the products.
⚗️ The Calculation
Total mass of reactants = mass of A + mass of B = 10 g + 15 g = 25 g
Total mass of products = 25 g
• Atoms are only rearranged during a reaction; no atom is created or lost.
• This is exactly why every chemical equation must be balanced — the number of atoms of each element must be equal on both sides.
Example: 2H₂ + O₂ → 2H₂O, where 4 g + 32 g = 36 g on each side.
❌ Why the other options are incorrect
✗ 10 g — this is only the mass of substance A; the mass of B has been left out entirely.
✗ 11 g — no chemical logic gives this figure; mass cannot fall from 25 g, as nothing escapes in a closed system.
✗ 16 g — again less than the total reactant mass; mass can never simply disappear in a chemical change.
📌 SSC & Railway Exam Facts
• Law of conservation of mass — Antoine Lavoisier, the father of modern chemistry.
• Law of constant (definite) proportions — Joseph Proust: a compound always has the same elements in the same mass ratio (water always H : O = 1 : 8).
• Dalton's atomic theory explained both these laws.
• A balanced equation is a direct statement of the law of conservation of mass.
• In reactions that give off a gas, the vessel must be closed, or the mass will seem to decrease.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 2 · Valency and chemical formulae

Q92. Which of the following molecules is diatomic in nature?

  1. (a) Phosphorus
  2. (b) Argon
  3. (c) Helium
  4. (d) Oxygen
Answer & solution

Answer: (d) Oxygen

Correct Answer: Option 4 — Oxygen
The atomicity of an element is the number of atoms present in one molecule of that element. An element whose molecule contains two atoms is called diatomic.
Oxygen exists in air as the molecule O₂, in which two oxygen atoms are joined by a double covalent bond. Its atomicity is therefore 2, making it diatomic. Oxygen is a non-metal with valency 2, and a single molecule of O₂ has a molecular mass of 32 u.
The common diatomic elements to remember are H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
❌ Why the other options are incorrect
✗ Phosphorus — its white form exists as P₄, so phosphorus is tetra-atomic (atomicity 4). Sulphur is likewise S₈, poly-atomic.
✗ Argon — a noble gas of group 18 with a complete octet; it does not bond even with itself, so it is monoatomic (Ar).
✗ Helium — also a noble gas, with a complete duplet of 2 electrons; it exists as single atoms and is monoatomic (He).
📌 SSC & Railway Exam Facts
• Atomicity: monoatomic — He, Ne, Ar, Kr, metals; diatomic — H₂, N₂, O₂, Cl₂; tri-atomic — O₃ (ozone); tetra-atomic — P₄; poly-atomic — S₈.
• Ozone (O₃) is an allotrope of oxygen and is tri-atomic.
• Noble gases are chemically inert because their outermost shell is completely filled.
• Molecular mass of O₂ = 32 u, of O₃ = 48 u, of N₂ = 28 u, of H₂ = 2 u.
• Oxygen makes up about 21% of air by volume and is the most abundant element in the Earth's crust.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 2 · Atomic mass and molecular mass

Q93. What is the calculated molecular mass of oxygen?

  1. (a) 30
  2. (b) 26
  3. (c) 28
  4. (d) 32
Answer & solution

Answer: (d) 32

Correct Answer: Option 4 — 32
The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of it. It is expressed in atomic mass units (u) and is a relative quantity, measured against 1/12 of the mass of one carbon-12 atom.
🔢 Calculation
• Oxygen gas exists as the diatomic molecule O₂.
• Atomic mass of one oxygen atom = 16 u.
• Molecular mass of O₂ = 2 × 16
• Molecular mass of O₂ = 32 u
The common error is to write 16, which is the atomic mass of a single oxygen atom, not the molecular mass of the gas. Remember that free oxygen never exists as single atoms under ordinary conditions.
❌ Why the other options are incorrect
✗ 30 — this is the molecular mass of nitric oxide (NO), 14 + 16, and also of ethane C₂H₆.
✗ 26 — this is the molecular mass of acetylene (C₂H₂), (2 × 12) + (2 × 1).
✗ 28 — this is the molecular mass of nitrogen gas (N₂), 2 × 14, and of carbon monoxide (CO), 12 + 16. It is the classic distractor for this question.
📌 SSC & Railway Exam Facts
• 1 u = 1/12 the mass of one C-12 atom.
• Useful molecular masses: H₂ = 2, N₂ = 28, O₂ = 32, O₃ = 48, Cl₂ = 71, CO₂ = 44, H₂O = 18, NH₃ = 17, CH₄ = 16, H₂SO₄ = 98.
• 1 mole of any substance contains 6.022 × 10²³ particles — Avogadro's number.
• The mass of 1 mole of O₂ = 32 g; 1 mole of any gas at STP occupies 22.4 litres.
• Gram molecular mass is the molecular mass expressed in grams.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Atomic mass and molecular mass

Q94. Which of the following is the correct unit mass of sodium?

  1. (a) 23u
  2. (b) 18u
  3. (c) 20u
  4. (d) 11u
Answer & solution

Answer: (a) 23u

Correct Answer: Option 1 — 23u
Sodium (Na) has atomic number 11, which means it has 11 protons and, in a neutral atom, 11 electrons. Its nucleus also contains 12 neutrons.
Atomic mass (mass number) = protons + neutrons = 11 + 12 = 23 u
So the atomic mass of sodium is 23 u, and its gram atomic mass is 23 g/mol. The unit u stands for the unified atomic mass unit, defined as 1/12 of the mass of one carbon-12 atom.
Sodium's electronic configuration is 2, 8, 1, so it has one valence electron, a valency of 1, and it belongs to Group 1 (alkali metals), Period 3.
❌ Why the other options are incorrect
✗ 18u — this is close to the atomic mass of water (18 u) and to the atomic number of argon (18); it is not sodium's mass.
✗ 20u — this is the atomic mass of neon (20 u) and the atomic mass of calcium's atomic number (20); not sodium.
✗ 11u — this is sodium's atomic number (11), a common trap; the atomic number is the proton count, not the mass.
📌 SSC & Railway Exam Facts
• Na: Z = 11, A = 23, neutrons = 12, configuration 2, 8, 1, valency 1.
• 1 u = 1/12 the mass of a carbon-12 atom ≈ 1.66 × 10⁻²⁷ kg.
• Common atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40.
• Sodium is stored in kerosene and gives a golden-yellow flame test.
• Molecular mass of NaCl = 23 + 35.5 = 58.5 u.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Valency and chemical formulae

Q95. Which of the following represents the correct formula for calcium oxide?

  1. (a) CaO
  2. (b) Ca₂O₃
  3. (c) CaO₂
  4. (d) Ca₂O
Answer & solution

Answer: (a) CaO

Correct Answer: Option 1 — CaO
A chemical formula is written by the criss-cross method, using the valency of each ion. Calcium is a Group 2 metal with valency 2, forming Ca²⁺; oxygen is a Group 16 non-metal with valency 2, forming the oxide ion O²⁻.
Criss-crossing the valencies gives Ca₂O₂, and dividing both subscripts by their HCF (2) gives the simplest formula, CaO. Chemically it makes perfect sense: one Ca²⁺ ion exactly balances one O²⁻ ion, so the compound is electrically neutral in a 1 : 1 ratio.
Calcium oxide (CaO) is the white solid commonly called quicklime or burnt lime. It is made by the calcination of limestone, CaCO₃ → (heat) CaO + CO₂, and it reacts vigorously with water to give slaked lime, CaO + H₂O → Ca(OH)₂, releasing a large amount of heat.
❌ Why the other options are incorrect
✗ Ca₂O₃ — this would require calcium to have a valency of 3; calcium belongs to Group 2 and is always divalent.
✗ CaO₂ — this is the formula of calcium peroxide, which contains the peroxide ion (O₂²⁻) and not the ordinary oxide ion; it is a different compound altogether.
✗ Ca₂O — this would mean calcium has a valency of 1, which is wrong, and the formula would not be electrically balanced.
📌 SSC & Railway Exam Facts
• CaO = quicklime; Ca(OH)₂ = slaked lime; CaCO₃ = limestone/marble/chalk; CaSO₄·2H₂O = gypsum.
• CaCO₃ → (heat) CaO + CO₂ is a decomposition reaction; CaO + H₂O → Ca(OH)₂ is an exothermic combination reaction.
• Limewater is a solution of Ca(OH)₂; it turns milky with CO₂ because insoluble CaCO₃ is formed.
• Valency: Group 1 = 1, Group 2 = 2, Group 13 = 3, Group 16 = 2, Group 17 = 1; noble gases = 0.
• The same criss-cross rule gives Na₂O, MgO, Al₂O₃, CO₂ and SO₂.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Valency and chemical formulae

Q96. What will be the atomicity of oxygen and phosphorous?

  1. (a) Diatomic and triatomic
  2. (b) Diatomic and diatomic
  3. (c) Diatomic and tetra-atomic
  4. (d) Diatomic and monoatomic
Answer & solution

Answer: (c) Diatomic and tetra-atomic

Correct Answer: Option 3 — Diatomic and tetra-atomic
Atomicity is the number of atoms present in one molecule of an element. On this basis elements are called monoatomic (1), diatomic (2), triatomic (3), tetra-atomic (4) or polyatomic.
Oxygen normally exists as the molecule O₂, which contains two atoms, so its atomicity is 2 — diatomic. Phosphorus, in its common white/yellow form, exists as the molecule P₄, a tetrahedral cluster of four atoms, so its atomicity is 4 — tetra-atomic.
Other standard values worth memorising: He, Ne, Ar and all noble gases — monoatomic; H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ — diatomic; O₃ (ozone) — triatomic; S₈ (sulphur) — polyatomic (octa-atomic).
❌ Why the other options are incorrect
✗ Diatomic and triatomic — oxygen is right, but phosphorus exists as P₄, not P₃; triatomic describes ozone, O₃.
✗ Diatomic and diatomic — phosphorus does not exist as P₂ under ordinary conditions; its stable molecule is P₄.
✗ Diatomic and monoatomic — monoatomic elements are the noble gases (He, Ne, Ar, Kr, Xe, Rn) and metals in the vapour state, certainly not phosphorus.
📌 SSC & Railway Exam Facts
• Atomicity = number of atoms in one molecule of an element.
• Monoatomic: noble gases; diatomic: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂; triatomic: O₃; tetra-atomic: P₄; polyatomic: S₈.
• Ozone (O₃) is an allotrope of oxygen and shields the Earth from harmful ultraviolet rays.
• White phosphorus (P₄) is stored under water because it catches fire spontaneously in air.
• Allotropes of carbon: diamond, graphite and fullerene (C₆₀); of sulphur: rhombic and monoclinic.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 3 · Valency and chemical formulae

Q97. Find the correct number of atoms for a glucose molecule.

  1. (a) 24
  2. (b) 18
  3. (c) 6
  4. (d) 12
Answer & solution

Answer: (a) 24

Correct Answer: Option 1 — 24
The molecular formula of glucose is C₆H₁₂O₆. A molecular formula tells us exactly how many atoms of each element are present in one molecule of the compound, so we simply add the subscripts:
• Carbon (C) = 6 atoms
• Hydrogen (H) = 12 atoms
• Oxygen (O) = 6 atoms
Total atoms = 6 + 12 + 6 = 24 atoms in one molecule of glucose.
Glucose is a monosaccharide (simple sugar) produced in photosynthesis and is the body's chief source of instant energy. Its molecular mass is 180 u (6×12 + 12×1 + 6×16), and its empirical formula is CH₂O.
❌ Why the other options are incorrect
✗ 18 — this is the total only of C + H (6 + 12); the 6 oxygen atoms have been left out.
✗ 6 — this is only the number of carbon atoms (or of oxygen atoms), not the total of all atoms.
✗ 12 — this is only the number of hydrogen atoms in the molecule.
📌 SSC & Railway Exam Facts
• Glucose = C₆H₁₂O₆, 24 atoms, molecular mass 180 u, empirical formula CH₂O.
• Sucrose = C₁₂H₂₂O₁₁ (45 atoms); fructose also has the formula C₆H₁₂O₆ — an isomer of glucose.
• Photosynthesis: 6CO₂ + 6H₂O → (sunlight, chlorophyll) C₆H₁₂O₆ + 6O₂.
• Empirical formula gives the simplest whole-number ratio; the molecular formula gives the actual number of atoms.
• One mole of any substance contains 6.022 × 10²³ particles — the Avogadro number.
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 2 · Valency and chemical formulae

Q98. What does the subscript in a chemical formula represent?

  1. (a) The charge of the ion
  2. (b) The oxidation state of the element
  3. (c) The number of atoms of each element in a molecule
  4. (d) The mass of the compound
Answer & solution

Answer: (c) The number of atoms of each element in a molecule

Correct Answer: Option 3 — The number of atoms of each element in a molecule
In a chemical formula, the small number written below and to the right of an element's symbol is called the subscript. It states how many atoms of that element are present in one molecule (or formula unit) of the substance. A subscript of 1 is never written — we write NaCl, not Na₁Cl₁.
For example, H₂SO₄ contains 2 hydrogen, 1 sulphur and 4 oxygen atoms. In Al₂(SO₄)₃ the subscript 3 outside the bracket multiplies everything inside it, giving 2 Al, 3 S and 12 O atoms — a favourite exam trap.
Do not confuse the subscript with the coefficient written in front of a formula: in 2H₂O the leading 2 counts whole molecules and is adjusted while balancing an equation, while the 2 in H₂ counts atoms and must never be changed.
❌ Why the other options are incorrect
✗ The charge of the ion — the charge of an ion is written as a superscript, e.g. Na⁺, Ca²⁺, SO₄²⁻, not as a subscript.
✗ The oxidation state of the element — oxidation state is shown by Roman numerals in brackets in the name, e.g. iron(III) chloride, or is calculated from the formula; the subscript does not give it.
✗ The mass of the compound — mass is obtained by adding the atomic masses of all the atoms (molecular mass in u); the subscript is only a count of atoms, not a mass.
📌 SSC & Railway Exam Facts
• Subscript = number of atoms; coefficient (written in front) = number of molecules.
• H₂O has 2 H + 1 O atoms; its molecular mass = 18 u.
• Ca(OH)₂ contains 1 Ca, 2 O and 2 H atoms.
• Charge is always a superscript: Cl⁻, SO₄²⁻, PO₄³⁻.
• A middle dot shows water of crystallisation: CuSO₄·5H₂O (blue vitriol), Na₂CO₃·10H₂O (washing soda).
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 3 · Atomic mass and molecular mass

Q99. The atomic mass of an element is usually expressed in which of the following units?

  1. (a) Grams per mole (g/mol)
  2. (b) Kilograms
  3. (c) Atomic mass units (amu)
  4. (d) Pounds
Answer & solution

Answer: (c) Atomic mass units (amu)

Correct Answer: Option 3 — Atomic mass units (amu)
Atoms are extraordinarily light — one hydrogen atom weighs about 1.67 × 10⁻²⁴ g — so grams and kilograms are hopelessly clumsy for them. Chemists therefore use a relative unit, the atomic mass unit (amu, symbol u).
Since 1961 the standard has been the carbon-12 atom: 1 u = 1/12 of the mass of one ¹²C atom ≈ 1.66 × 10⁻²⁷ kg. Because the scale is relative, an atomic mass such as 16 u simply means that the atom is 16 times heavier than one-twelfth of a carbon-12 atom.
Examples: H = 1 u, C = 12 u, N = 14 u, O = 16 u, Na = 23 u, Cl = 35.5 u. The modern symbol is u (unified atomic mass unit), also called the dalton (Da).
❌ Why the other options are incorrect
✗ Grams per mole (g/mol) — this is the unit of molar mass, the mass of one mole (6.022 × 10²³ particles), not of a single atom. It is numerically equal to the atomic mass but is a different physical quantity.
✗ Kilograms — the SI unit of mass, far too large here; an atom's mass in kg comes to about 10⁻²⁶–10⁻²⁷, an unusable figure for everyday chemistry.
✗ Pounds — an imperial (FPS) unit used for commercial weighing; it has no role in atomic measurement.
📌 SSC & Railway Exam Facts
• 1 u = 1/12 the mass of a ¹²C atom = 1.66 × 10⁻²⁷ kg.
• Avogadro's number = 6.022 × 10²³; one mole of any substance contains this many particles.
• Molar mass in g/mol is numerically equal to the atomic or molecular mass in u.
• Earlier reference standards were hydrogen (Dalton) and then oxygen; carbon-12 was adopted in 1961.
• The atomic mass unit is also called the dalton (Da), named after John Dalton.
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 3 · Dalton's atomic theory

Q100. The smallest particle of an element that retains its properties is called a/an:

  1. (a) atom
  2. (b) compound
  3. (c) electron
  4. (d) proton
Answer & solution

Answer: (a) atom

Correct Answer: Option 1 — atom
An atom is the smallest particle of an element that takes part in a chemical reaction and still retains all the chemical properties of that element. The word comes from the Greek *a-tomio*, meaning "indivisible", and the idea was first given scientific shape by John Dalton (1808) in his atomic theory.
An atom is built from three fundamental particles — protons (+) and neutrons (neutral) inside the nucleus, and electrons (−) revolving around it. If an atom is broken down into these particles, the identity and the properties of the element are lost: a lone proton is no longer "carbon" or "oxygen".
The corresponding smallest particle of a substance that can exist independently is the molecule.
❌ Why the other options are incorrect
✗ compound — a compound is formed from two or more different elements chemically combined in a fixed ratio; it is not a particle of a single element at all.
✗ electron — a sub-atomic particle carrying a negative charge. Electrons are identical in every element, so an electron by itself carries none of an element's characteristic properties.
✗ proton — also a sub-atomic particle, positively charged. Its number decides which element we have, but a free proton on its own does not show the properties of that element.
📌 SSC & Railway Exam Facts
• Atom = smallest particle of an element; molecule = smallest particle of a substance that can exist freely.
• Dalton's atomic theory (1808) first stated that matter is made of indivisible atoms.
• Atomic radius is measured in nanometres (1 nm = 10⁻⁹ m); atoms are about 10⁻¹⁰ m across.
• Atomic number (Z) = number of protons; mass number (A) = protons + neutrons.
• The word atom goes back to Democritus; the modern symbols of elements were given by Berzelius.

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